Conventional fluid filters, such as oil filters, are basically mechanical strainers which include a filter element having pores which trap and segregate dirt from the fluid. Since the flow through the filter is a function of the pore size, filter flow will decrease as the filter pack becomes clogged with dirt. Since the filtration system must remove dirt at the same rate at which it enters the oil, a clogged conventional pack cannot process enough oil to keep the dirt level of the oil at a satisfactory level. A further disadvantage of some mechanical strainer type filters is that they tend to remove oil additives. Furthermore, the additives may be depleted to some extent by acting upon trapped dirt in the filter and are rendered ineffective for their intended purpose on a working surface in the engine.
Prior art centrifugal filters have been proposed which do not act as mechanical strainers but, rather, remove contaminants from a fluid by centrifuging. For example, such a filter is shown in U.S. Pat. No. 3,432,091 granted to Beazley. In the Beazley patent, there is illustrated a hollow rotor which is rotatably mounted on a spindle. The spindle has an axial passageway which conducts oil into the interior of the rotor. Tangentially directed outlet ports are provided in the rotor so that the rotor is rotated upon issuance of the fluid therefrom. Solids, such as dirt, are centrifuged to the sidewalls of the rotor and the dirt may be later removed by disassembling the rotor and scraping the filter cake from the sidewalls.
Such centrifugal filters have oil inlets and outlets through the base of the filter, since access to the rotor for cleaning purposes is provided by removing a shroud cover and by then removing the rotor from the spindle. Such a maintenance operation is frequently not necessary, but the user of the filter has no way of knowing this until after the filter is disassembled.